1. Field of the Invention
The present invention generally relates to glass filler compositions and methods for their preparation and use. More particularly, the present invention relates to glass filler compositions which can be bonded to silicon carbide (SiC) foam surfaces and provide surfaces suitable for the deposition of additional SiC materials.
2. Description of Related Art
Lightweight mirrors having suitable reflective surfaces are frequently utilized in space-based optical systems. Such optical systems require mirrors which are resistant to distortion caused by gravitational sag effects. Due to its low density, high stiffness and excellent polishability, SiC foam is particularly suitable for these applications. Lightweight SiC foam in the form of mirrors is also useful in cryogenic optical systems where the low heat capacity of SiC foam aids in cooling and maintaining the mirrors at cryogenic temperatures.
Typically, SiC foam is prepared by depositing a layer of SiC, using chemical vapor deposition techniques, onto a carbon foam substrate. After the SiC layer is deposited, the carbon foam is removed pyrolytically to leave a monolithic SiC foam core. To provide a densified SiC surface for the application of polishable SiC material, a thin layer of SiC cement is then normally applied over the foam. This forms a laminated facesheet which acts as a substrate for the final thin coating of polishable chemical vapor deposited SiC. The thin layer of SiC is then machined, polished and, at times coated with a reflective thin film, to provide a reflective mirror surface having the desired optical quality. Light weight mirrors comprising SiC foam cores with SiC cement facesheets are described in U.S. Pat. No. 4,856,887.
Such SiC foam core mirrors must have facesheets which withstand continual temperature cycling between ambient and cryogenic temperatures and in some instances elevated temperatures. In considering the demands for dimensional stability at a variety of temperatures, at the least, the facesheet material and the SiC foam must have coefficients of thermal expansion which are identical. Additionally, the facesheet material preferentially forms a strong bond to the SiC foam to decrease the possibility of delamination during temperature cycling and a subsequent loss in the facesheet optical quality.
One problem with prior SiC cement facesheets is their tendency to delaminate from the SiC foam core because the SiC cement and SiC foam core are not sufficiently bonded together. Accordingly, there is a need to provide materials for SiC facesheets which will integrally bond to the SiC foam core. There is additionally a need to provide materials for SiC facesheets with a substantially equivalent coefficient of thermal expansion as the SiC foam mirrors.